Trouble with Oncidiums - Virus or pest?
 

My Miltassia and Degarmoara have been infected with this rust-like stuff on their leaves. It has only appeared on older leaves. Both plants are actively growing, and the new leaves are fine (so far....) The lighter coloured flecks appear like dust and can be rubbed off. I thought spider mites at first, but there is no movement and no webbing. The rust discolouration is right in the leaf. It is most prominent in the folds of the leaf at the base, and where there is the odd pleated spot. But today I also found some on the underside at a tip. I've been removing infected leaves and spraying with an equivalent to Physan. However, it keeps appearing. Any ideas? Help please!

I had that, and I believe it could be spider mites. It looks like rust when you wipe it off right?

also, never had webbing or movement either, so I found it strange as well.

The rust colouring doesn't seem to come off. When I rub a paper towel on it, the loose lighter coloured flecks come off. I have to rub really hard to get a rust colour on the towel. Hard enough that the leaves are damaged, so I think I'm actually transfering leaf cells.
I tried using a pest spray yesterday. I'm trying to get some neem oil just in case. I've been using Physan because it reminds me of rust fungus that I sometimes get on roses. Any other suggestions to try? These plants are two of my "originals" and I'd really hate to lose them.

Try wiping the leaves with soapy water and do not rinse them afterwards. If it is a critter, that will knock them back considerably.